1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a capacitor of a semiconductor device with improved characteristics by removing water-soluble oxides formed on the storage node oxide film.
2. Description of the Related Art
In recent years, high integration of semiconductor memory devices has resulted in a decrease in the design rule of the devices and a reduction in the area occupied by capacitors. Thus, many difficulties have been encountered in ensuring sufficient capacity of capacitors. In an attempt to solve these difficulties, a number of studies are being undertaken to increase the height of storage nodes, for example, the step of storage node oxide films. However, a major limitation in increasing the step of storage node oxide films is an insufficient process margin. Another attempt to increase the capacity of capacitors is the use of both inner and outer sides of cylindrical capacitors fabricated by removal of storage node oxide films. In the case where storage node oxide films are removed by wet etching, however, capacitors may fall down or become uprooted. To solve the problems caused by wet etching, there have been suggestions to form storage node oxide films into dual oxide films.
FIG. 1 is a view illustrating a capacitor of a semiconductor device according to the prior art.
Referring to FIG. 1, first, a transistor and a bit line (not shown) are formed on a semiconductor substrate 110, and an interlayer insulating film 110 is formed thereon. Contact plugs are formed in such a manner that they are connected to active regions of the semiconductor substrate 110 through the interlayer insulating film 110. The interlayer insulating film 110 may be composed of a plasma enhanced TEOS (PETEOS) film, and the contact plugs 120 may be formed by embedding a conductive material in contact holes. Subsequently, a storage node oxide film 130 is formed on the interlayer insulating film 110 and the contact plugs 120.
The storage node oxide film 130 is selectively etched in such a way that the contact plugs 120 are exposed to form capacitor holes. A material for a storage node electrode is embedded in the capacitor holes, and flattened by etchback, chemical mechanical polishing or the like to form a storage node electrode 140.
On the other hand, the storage node oxide film 130 may be composed of a single film of an undoped silicate glass (USG), high-density plasma (HDP) or TEOS oxide film. Alternatively, the storage node oxide film 130 may be composed of a double film consisting of films having different wet etching characteristics, for example, a double film consisting of a first capacitor sacrificial insulating film composed of a phosphorus silicate glass (PSG) oxide film and a PETEOS film formed on the PSG film.
If a PSG film is used as the storage node oxide film 130, phosphorus (P), which is an impurity present in the PSG film, is continuously diffused out according to the changes in the concentration of phosphorus (P) in a thin film and increasing exposure time in air after formation of the PSG film, and reacts with moisture present in the air to form water-soluble oxides on the surface of the PSG film.
FIG. 2 is a graph showing changes in the composition of a thin film as a function of time after formation of the storage node oxide film in accordance with the prior art.
FIGS. 3a and 3b show the formation of water-soluble oxides on the surface of the PSG film with increasing the exposure time in the air after formation of the storage node oxide film in accordance with the prior art.
FIGS. 4 to 6 are views explaining problems arising due to the presence of the water-soluble oxides shown in FIG. 3.
Referring to FIG. 2, when a PSG film is used to form a storage node oxide film by the prior art, the concentration of phosphorus (P) that can be diffused out in a thin film increases with the passage of time after formation of the PSG film. The diffused-out phosphorus (P) is not observed in the initial stage of the formation of the PSG film (FIG. 3a), but reacts with moisture present in air with the passage of time to form water-soluble oxides 300 on the surface of the PSG film.
In the formation of a multiple thin film consisting of films having different wet etching selectivity characteristics, the water-soluble oxides 300 have a considerably different wet etching selectivity from that of a normal interface between the PSG film and a subsequent thin film. As a result, tunneling 400 takes place to form storage node bridges, as shown in FIG. 4. Referring to FIG. 5, steps 500 caused by the water-soluble compounds induce formation of serious steps in the subsequent processing after deposition, causing defects upon pattern formation.
To prevent the formation of the water-soluble compounds, subsequent processing is performed immediately after formation of the PSG film to minimize the exposure time in air, or a wafer exposed to ambient air for a long time after formation of the PSG film is subjected to high-pressure cleaning using deionized water to inhibit the formation of water-soluble compounds. However, the former is unsuitable to fabricate capacitors on an industrial scale. The latter is a process wherein water-soluble compounds are removed using water supplied by means of simple rotation instead of thermal treatment upon wafer drying. A problem of this process is that defect sources 600 are left on the wafer edges with the passage of time, as shown in FIG. 6.